1. Field of the Invention
The present invention relates to a calender.
2. Description of the Prior Art
In the rollers described in the preamble, the bearing pockets rest against the inside circumference of the hollow cylinder with an edge which is sealed all around. The edge forms essentially sealed hydrostatic pressure chambers. The pressure of the chambers acts against the inside circumference of the hollow cylinder and produces the linear force in the roll nip. The pressure fluid is supplied via a first feed line and constantly flows to the outside over the edge of the bearing pockets, causing a bearing fluid film to form between the edge and the inside circumference of the hollow cylinder, preventing metal-to-metal contact. In order for the bearing pockets to be sealed and for pressure to build up in the pockets, the edge has to be pressed against the inside circumference of the hollow cylinder with a certain force. This is accomplished with the pressure in the sealed cylinder space. The sealed cylinder space is structured as a piston/cylinder unit in the supporting element, the force of which presses the supporting element and therefore the edge against the inside circumference of the hollow cylinder. The pressure fluid which acts on the cylinder space is supplied via a second feed line. The bearing pockets have pressure fluid at different pressures applied to them, individually or in various groups, in order to be able to adjust a linear force profile of the desired type along the roll nip. In practice the cylinder spaces are generally connected with a common feed line at a single pressure, which feed line is dimensioned so as to maintain sufficient contact.
A roller with two feed lines to the supporting elements is known from German Patent 38 20 974 A1. Such a roller has already been used in a calender, which thereby represents the state of the art. An important characteristic of the roller according to German Patent 38 20 974 C2 is that the bearing pockets are connected with the first feed line without a throttle, i.e. that the pressure fluid can pass over into the bearing pockets from the space under the supporting element, through the inner channels which are present in the supporting element, and have a cross-section which does not cause any significant pressure drop in normal operation.
The rollers of the prior art are also used in calenders with plastic-coated roller mantles. The plastic coatings are significantly more sensitive, mechanically and thermally, than a metal roller surface. If web problems occur, such a soft roller coating is therefore easily damaged. A web problem is, for example, a break in the web, which has the result that a roller with a plastic coating suddenly comes into contact with a hot counter-roller, something that the plastic cannot tolerate. In normal operation, the roller surface is protected from the hot counter-roller by the cooler web, for example the paper web. Doubling of the web can also damage the surface of the plastic coating, if the linear forces are high, by pressing the web into the coating.
In the case of such web problems, rapid opening of the roll nip due to the danger of damage to the roller coating is absolutely necessary.
During normal production of the calender, the cross-head of the roller is deflected away from the roll nip, due to the forces of the supporting elements which support the roller mantle. In the case of calanders with a large width, the amount of deflection is several centimeters in the center. The change in distance between the cross-head and the inside circumference of the roller mantle is overcome by moving the supporting elements out in a direction perpendicular to the cross-head. The volume of pressure fluid enclosed under the supporting element is increased by the amount which corresponds to this movement path.
This also holds true for calenders with deflection-controlled rollers with a so-called inside stroke. Here, to adjust the contact with a counter-roller, rather than moving the position of the cross-head of such a roller in its mounting on the roller stand, the hollow cylinder is moved perpendicular to its axis, by way of its so-called "inside stroke," to release the roll nip (as shown in German Patent AS 22 54 392). The supporting elements move in towards or out from the cross-head, so that the roller mantle can follow in these directions, parallel to itself. In rollers with inside stroke, the hollow cylinder is not mounted on the cross-head at its ends, but rather merely guided along the cross-head, in the plane of effect, perpendicular to its axis. The problem with that arrangement is that when the roller is in operating position, and when the supporting elements are moved out accordingly, large volumes of pressure fluid are enclosed under them exists.
The terms "at the bottom" and "at the top" as used herein are based on the orientation of a roll nip above the roller, as shown in FIG. 1.
If stress on the roll nip is supposed to be relieved quickly, these volumes of pressure fluid must be able to escape from the supporting element very quickly. This holds true both for mounted rollers, on which the cross-head is only deflected and only the axially inside supporting elements must be moved in again when the cross-head is retracted, and for rollers with inside stroke, on which the cross-head not only retracts but also moves crosswise relative to the hollow cylinder, as a whole.
The volumes which can escape are not very large and amount to approximately 942 cm.sup.3 for 3 cm displacement of a single supporting element with a diameter of 200 mm, for example. This is true whether the displacement occurs as the result of deflection of the cross-head in the center, or as the result of crosswise movement with inside stroke. However, if stress on the roll nip is supposed to be relieved in at most 0.5 seconds, this corresponds to an instantaneous oil flow of 113,000 cm.sup.3 /min.
Each supporting element, i.e. each group of elements, has to be connected with the pressure supply and control system outside the roller via a separate line for the type of rollers in question. If each individual supporting element is controlled individually or in groups, and the stability of the cross-head is not allowed to be impaired by large inside channels, only relatively small pressure line cross-sections are possible within the cross-head, for reasons of space. Fast relief of the volumes of pressure fluid to be released cannot be accomplished with these pressure line cross-sections in the required amount of time and at the available pressures.
In the known calender stacks equipped with conventional rollers, where hard rollers alternate with soft rollers, usually so-called paper rollers, this problem has been known for a long time and has already been solved in different ways, for example in that the rollers are allowed to drop in their side mounts, to a stop, if web problems occur, in such a way that all of the roll nips of the calender are opened at the same time (as shown in DE-OS 20 10 322).
There are also already solutions for calenders with deflection-controlled rollers, although these have paid no attention to the problem of the line cross-sections (DE-AS 23 20 519).
In German Patent 28 48 021 B1, a roller of another type is shown, for which a quick-relief device has been included. However, this roller uses the "Nipco" principle, in which every supporting element is charged with fluid at only one pressure. The piston-like supporting element is seated to move back and forth in a cylinder bore of the cross-head, into which the pressure fluid is guided. A throttle bore is formed in the supporting element, which connects the cylinder chamber under the supporting element with the bearing pockets. The pressure fluid therefore first enters the cylinder chamber under the supporting element and presses the latter against the inside circumference of the hollow cylinder. The same pressure fluid gets into the bearing pockets through the throttle bore, and performs the function of hydrostatic support of the hollow cylinder by the supporting element. In German Patent 28 48 021 B1, a valve interrupts the feed of pressure fluid to all the supporting elements at the same time. This eliminates the contact pressure, but also the hydrostatic support and lubrication of each individual supporting element relative to the inside circumference of the hollow cylinder.